5 research outputs found
Improving predictions of critical shear stress in gravel bed rivers: Identifying the onset of sediment transport and quantifying sediment structure
Understanding when gravel moves in river beds is essential for a range of different applications but is still surprisingly hard to predict. Here we consider how our ability to predict critical shear stress (Ï c ) is being improved by recent advances in two areas: (1) identifying the onset of bedload transport; and (2) quantifying grainâscale gravel bed structure. This paper addresses these areas through both an inâdepth review and a comparison of new datasets of gravel structure collected using three different methods. We focus on advances in these two areas because of the need to understand how the conditions for sediment entrainment vary spatially and temporally, and because spatial and temporal changes in grainâscale structure are likely to be a major driver of changes in Ï c . We use data collected from a small gravelâbed stream using direct fieldâbased measurements, terrestrial laser scanning (TLS) and computed tomography (CT) scanning, which is the first time that these methods have been directly compared. Using each method, we measure structureârelevant metrics including grain size distribution, grain protrusion and fine matrix content. We find that all three methods produce consistent measures of grain size, but that there is less agreement between measurements of grain protrusion and fine matrix content